The present invention relates to a semiconductor laser device of ridge waveguide type. More particularly, the present invention relates to a semiconductor laser device of ridge waveguide type which has a desirably controlled half-width value θ// of a far field pattern (FFP) in a direction horizontal to a hetero interface, exhibits good laser characteristics during high-output operation and merely requires a low driving voltage.
Semiconductor laser devices of ridge waveguide type, including those which are based on GaAs or InP for long wavelengths and a nitride based III-V group compound for short wavelengths, find use in a various application areas because they are easy to manufacture.
The semiconductor laser device of ridge waveguide type belongs to the category of index guided device. It has an upper portion of an upper cladding layer and a contact layer, both resembling a striped-shaped ridge. The ridge is formed such that an insulating film covers both sides of the ridge and the upper cladding layer extending sideward from the base of the ridge. This insulating film functions as a layer to constrict electric current and provides an effective refractive index difference in the lateral direction for mode control.
An explanation is given below, with reference to FIG. 11, of the structure of a related-art nitride based III-V group compound semiconductor laser device of ridge waveguide type which emits light with a wavelength of about 410 nm. This laser device is referred to as “nitride based semiconductor laser device” hereinafter.
FIG. 11 shows a related-art nitride based semiconductor laser device of ridge waveguide type 10 has basically a stacked structure in which a plularity of layers are stacked on a sapphire substrate 12. The plularity of layers stacked on the sapphire substrate 12 are a laterally grown GaN layer 14, an n-GaN contact layer 16, an n-AlGaN cladding layer 18, an active layer 20, a p-AlGaN cladding layer 22, and a p-GaN contact layer 24.
In the stacked structure, the upper portion of the p-AlGaN cladding layer 22 and the p-GaN contact layer 24 are formed as a striped-shaped ridge 26. A mesa structure extending in the same direction as the ridge 26 is formed by the upper portion of the n-GaN contact layer 16, the n-AlGaN cladding layer 18, the active layer 20, and the remaining portion 22a of the p-AlGaN cladding layer 22.
The ridge 26 has a width (W) of about 1.7 μm. The remaining portion 22a of the p-AlGaN cladding layer 22 which extends sideward from the base of the ridge 26 has a thickness (T) of about 0.17 μm.
An insulating film 28 of SiO2 (about 2000 Å thick) is formed on both sides of the ridge 26, the side of the mesa structure above the p-AlGaN cladding layer 22 extending sideward from the base of the ridge 26, and the n-AlGaN contact layer 16.
On the insulating film 28 is formed a p-side electrode 30, which is in contact with the p-GaN contact layer 24 through a window in the insulating film 28. On the n-GaN contact layer 16 is formed an n-side electrode 32.
The nitride based semiconductor laser device of ridge waveguide type 10 mentioned above is considered as a highly efficient one because the insulating film 28 covering both sides of the ridge 26 is transparent to the emitted laser beam with little waveguide loss and the threshold current is small.
In the meantime, as its application areas expand, the nitride based semiconductor laser device of ridge waveguide type is required to have a higher kink level so that it maintains good characteristic property for light output vs. injected current throughout the region up to the high-output level. It is also required to have a larger half-width value θ// of a far field pattern (FFP) in a direction horizontal to the hetero interface.
For example, in the case where the nitride based semiconductor laser device is used as a light source of an optical pickup, it is required to have a larger half-width value θ//.
The results of the present inventors' investigation revealed that the value of θ// is related closely with the difference (Δn) of effective refractive index of the ridge waveguide, as shown in FIG. 12. In order to obtain a larger value of θ//, it is necessary to have a larger value of Δn. Incidentally, the difference (Δn) of effective refractive index of the ridge waveguide is defined as neff1–neff2 or a difference between neff1 which is the effective refractive index of the ridge for the oscillation wavelength and neff2 which is the effective refractive index of the ridge's side, as shown in FIG. 11. Closed and open circles in FIG. 12 denote the values obtained by experiments.
Unfortunately, any attempt to increase the value of Δn ends up with a narrow cutoff ridge width of high-order horizontal lateral mode. The cutoff ridge width of high-order horizontal lateral mode is defined as a ridge width which gives rise to no high-order horizontal lateral mode. When the ridge width is larger than the cutoff ridge width, the horizontal lateral mode tends to shift from the fundamental mode to the primary high-order mode at the time of laser oscillation.
When a hybrid mode consisting of the fundamental horizontal lateral mode and the high-order horizontal lateral mode occurs, a kink occurs in the light output-injected current characteristics, as shown in FIG. 13. The result is a deterioration in the laser characteristics at the time of high-output operation.
The foregoing holds true particular for the nitride based semiconductor laser device of ridge wave-guide type, which has a small value of Δn and a short oscillation wavelength and hence has a narrow cutoff ridge width of high-order horizontal lateral mode, as shown in FIG. 14. FIG. 14 is a graph showing the relation between the value of Δn and the cutoff ridge width in the case where the GaN layer has a refractive index of 2.504 and an oscillation wavelength (λ) of 400 nm. Δn stands for the difference between the effective refractive index of the ridge and the effective refractive index of the ridge's side. For example, if the value of Δn is 0.005 to 0.01, the ridge width should be reduced to about 1 μm so that the ridge width is smaller than the cutoff ridge width.
As mentioned above, any attempt to increase the value of Δn, thereby increasing the value of θ//, ends up with a decreased cutoff ridge width, which leads to a deterioration in laser characteristics at the time of high-output operation. In other words, there is a trade-off for ridge width between the value of θ// and the laser characteristics at the time of high-output operation.
Moreover, the nitride based semiconductor laser device of ridge waveguide type has found an increasing use in the area of portable machines. The one for this purpose is required to have a lower drive voltage. One way to reduce the drive voltage is to increase the ridge width so that the contact area between the contact layer and the p-side electrode is increased. However, this suffers the disadvantage that the ridge width exceeds the cutoff ridge width, resulting in a deterioration in the laser characteristics at the time of high-output operation. In other words, there is a trade-off for the ridge width between the reduced drive voltage and the improved laser characteristics at the time of high-output operation.
The foregoing indicates that reducing the ridge width, thereby improving the laser characteristics at the time of high-output operation, contradicts increasing the value of θ// and decreasing the drive voltage.
As mentioned above, the related-art nitride based semiconductor laser device poses several problems. That is, it does not permit the ridge width to be decreased appreciably in order to keep its drive voltage low. Also, it has a ridge width lager than its cutoff ridge width, which prevents the kink level from being raised to a desired high level in the light output-injected current characteristics. The result is that the value of Δn is small and the value of θ// is also small.
The foregoing is applicable not only to nitride based semiconductor laser devices but also to any semiconductor laser devices (such as GaAs and InP) of ridge waveguide type for longer wavelengths.